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It's not that hard. You only need a few rules to make it work.

* Manage your own VPN and don't depend on the vendor's solution. * Only use base level services that are available on every cloud you want to leverage. Which probably means you're on a container based system and not using anything like Lambda.

Then a service being in another cloud just doesn't matter. You're always making requests via your VPN anyway, if it's in a local cloud or remote cloud it doesn't matter.

You'll feel the egress pricing if you integrate cross cloud services that are chatty. So it really depends on your immediate goals.

If you just want redundancy then you need to keep the resources standing in the backup cloud/AZ and just move your entry point via DNS, in which case you don't really need "cross cloud", just the ability to provision else where.